Glossary of Terminology

Beam angle
Angle in the plane through the beam axis over which
the luminous intensity drops to a stated percentage
(e.g. 50%) of its peak intensity.

Attribute of visual sensation according to which an area
appears to emit more or less light. Brightness according
to the definition is also an attribute of color.

Color appearance
General expression for the color impression received
when looking at a light source.

Color change
The facility to change the color temperature of a lighting
installation makes it possible to create a more comfortable
working environment or to introduce dynamic lighting

Color rendering
Ability of a light source to render colors naturally,
without distorting the hues seen under a full spectrum
radiator (like daylight or incandescent lamps). The color
rendering index (CRI) ranges from 0 to 100.
Color rendering index CRI
See Color rendering.
Symbol: Ra

Color temperature
Temperature in kelvin of a full spectrum radiator closest
to the color appearance of a light source at the same
Unit: kelvin, K
Symbol: Tc
Contrast C (between two parts of a visual field)
The relevant luminance difference between two parts
in accordance with the formula:
C=L1 – L2
Where the size of the two parts differs greatly and
L1 = luminance of the smallest part (object)
L2 = luminance of the largest part (background).

Luminous efficacy
The quantity of light that a light source emits per unit
of electrical energy consumed. Notice that the luminous
efficacy can be specified for a light source or for a lighting
system (lamp and ballast). The luminous efficacy of a
system is lower than that of the light source.
Unit: lm/W

Luminous flux
The total light output emitted by a light source. Also
the total light falling on a surface. The light output of a
light source is measured in lumen.
Unit: lumen
Symbol: ø

Luminous intensity
The luminous flux in a given direction, e.g. from a
floodlight or projector.
Unit: candela (cd)

The luminous flux density at the surface being lit. The
illuminance in the full summer sun is approx. 100,000
lux. Recommended illuminances for work places range
from 200 lux for rough work to 2000 lux for detailed or
critical work.
Unit: lux (lx) = lm/m2
Symbol: E

Power factor
Ratio of the circuit power in watts over the product of
the root mean square values of voltage and current. For
sinusoidal waveforms, it is equal to the cosine of the
angle of phase difference between voltage and current.
For electronic ballasts the power factor is 0.95pf so
that no additional phase compensation is required.

Red, green and blue, used to specify the colors of a
light source.

Efficiency and efficacy
The “efficacy” of a lighting fixture is usually expressed
as the amount of light (in lumens) produced by a certain
amount of electricity (in watts). Lighting fixture “efficiency”
is the ratio of the total lumens exiting the fixture to the
total lumens produced by the light source. Efficiency
is also used to discuss the broader concept of using
resources efficiently. What to remember:
specified fixture that you are looking at the
appropriate fixture data versus the LED
source data.

For traditional lighting sources lifetime is reported as
“mean time to failure,” which means the average time it
takes for some percentage of lamps (usually 50%) to fail
completely. With a LED source, light output diminishes
over time.Therefore for LED sources, lifetime typically
refers to the useful life of the light output, which is
referred to as lumen maintenance.